Use Multiple Google Accounts Simultaneously in Google Chrome

When you open a window in incognito mode (Ctrl+Shift+N), your cookies from the standard session are no longer available and you can easily log in to a new Google account.

Chrome's help center has more details about this feature. "Webpages that you open and files downloaded while you are incognito won't be logged in your browsing and download histories; all new cookies are deleted after you close the incognito window. You can browse normally and in incognito mode at the same time by using separate windows." Michael Jeffrey explains that "incognito mode is a window-level mode - all pages viewed within this window are not persisted to the user's history, and incognito pages use a temporary cookie store that is blank at the start of the incognito session".

When you close the incognito window, all the cookies are deleted so you need to log in again the next time you browse in incognito mode. This trick can be used for any other services that allow multiple accounts, but you can only use two accounts at the same time: one in a standard window and the other one in incognito mode.